1. Field of the Invention
This invention relates to catalytic combustion of a fuel, in particular, a process for catalytic combustion of natural gas.
Catalytic combustion, while a mature technology for a volatile organic compound incineration and gasoline or heavy fuels, has not been commercially developed for utilization of natural gas. Catalytic combustion is also the only combustion technology capable of producing nitrogen oxide emissions consistently below 10-ppm levels during the lean combustion of natural gas in small (1-5 MW) gas turbine engines. Several aspects of the process of catalytic combustion make the development of durable, high-temperature catalysts difficult for natural gas fuel. Firstly, the rate of diffusional convection of methane is comparable to the rate of convection cooling and, thus, the surface temperatures of an active catalyst can approach adiabatic temperatures. Secondly, there are no catalytic and substrate materials currently available that can long withstand the adiabatic temperatures required for efficient operation of gas turbines (typically greater than 1200.degree. C.). Finally, methane is the most difficult fuel to combust with a catalyst, and very active materials such as palladium oxide (PdO) must be used to sustain combustion under the desired fully premixed inlet conditions. Consequently, some methods for moderating catalyst wall temperatures during the early stages of premixed lean catalytic combustion are required so that the active catalysts have a long lifetime.
The catalytic combustion process and specific catalytic combustor configurations described herein take advantage of several concepts for preventing the overheating of catalysts during the catalytic combustion of natural gas. Each of these concepts are used and combined as stages in several combustor configurations. Specific catalytic materials and substrate materials are identified and described that are suitable for use in such catalytic combustors.
2. Description of Prior Art
Currently, advanced commercial combustors that are designed for low NO.sub.x emissions are lean, premixed (also known as dry low NO.sub.x) burners. A cluster of fuel nozzles are typically arranged in an annular pattern. They alternately fluctuate the fuel/air ratio to stabilize the flame while maintaining extremely lean conditions. Conventional burners use hotter flames (1800.degree. C.) with a large amount of secondary air mixed in downstream to lower the temperature to target levels. Such burners produce unacceptable NO.sub.x levels (50 ppm). The lean premixed burners currently available use a much lower amount of secondary air and can achieve lower NO.sub.x levels (about 10 ppm).
Catalytic combustors also use premixed fuel and air with or without an upstream flame preheater, and with or without downstream secondary air, depending upon inlet temperature, air throughput, and targeted outlet temperatures. The fuel and oxygen react on the surfaces of catalytically-active components which are dispersed over a high surface area to volume monolithic substrate structure. One or more monoliths are used in series to tailor component material properties with mean catalyst temperatures.
There are a number of reviews and published papers on the selection and use of catalytic combustion materials. See (1) Design Criteria for Stationary Source Catalytic Combustion Systems, J. P. Kesselring, Acurex/Energy and Environmental Division, Report to the Environmental Protection Agency EPA-600/7-79-181 (August 1979); (2) Applied Catalysis 7, D. L. Trimm, Catalytic Combustion (Review), (1983), pp. 249-82; (3) Recent Progress in High Temperature Catalytic Combustion, H. Arai and H. Machida, Catalysis Today, 10 (1991) pp. 81-95; (4) Catalytic Materials for High-Temperature Combustion, Marcus F. M. Zwinkels, et al., Catalysis Review--Science and Engineering, Vol. 35(3), pp. 319-358 (1993); (5) Research and Development on High Temperature Catalytic Combustion, Hiromichi Arai, et al, Catalysis Today, Vol. 26, pp. 217-221 (1995); (6) Catalysis in Combustion, Advanced Combustion Methods, J. P. Kesselring, F. J. Weinberg, ed., Chpt. 4, pp. 237-75 (Academic Press, 1986); (7) Catalytic Combustion for Ultra-Low Emissions, Robert J. Farrauto, et al., Materials Research Society Symposium, Materials Research Society, Vol. 344, pp. 101-120 (1994). In addition, several recent articles describe specific attempts to develop combustion catalysts from natural gas. See (1) Catalytic Combustion Technology to Achieve Ultra Low NO.sub.x Emissions: Catalyst Design and Performance Characteristics, Ralph A. Dalla Betta, et al, Catalysis Today, Vol. 26, pp. 329-335 (1995); (2) Development of a High-Temperature Combustion Catalyst System and Prototype Catalytic Combustor Turbine Test Results, H. Sadamori, et al., Catalysis Today, Vol. 26, pp. 337-344 (1995); (3) Development of a Hybrid Catalytic Combustor for a 1300.degree. C. Class Gas Turbine, Tomiak Furuya, et al., Catalysis Today, Vol. 26, pp. 345-350 (1995); (4) Development of a Low NO.sub.x Catalytic Combustor for a Gas Turbine, Yasushi Ozawa, et al., Catalysis Today, Vol. 26, pp. 351-357 (1995).
As previously stated, the primary technical problem associated with catalytic combustion of natural gas is the high diffusivity of methane in the premixed fuel and air stream which causes the reaction rate at the catalyst surface to match the rate of heat dissipation by conduction to the premixed gas in such a way that surface temperatures approach the adiabatic temperature. One solution to this problem is the use of a diffusion barrier layer (DBL) at least partially covering the catalyst, thereby placing a restriction on the rate of convective diffusion of natural gas fuel components to the hot, active catalyst surface while simultaneously not impeding the rate of cooling. This method also has the potential for lowering catalyst wall temperatures to tolerable levels. See, for example, U.S. Pat. No. 5,405,260 which teaches the use of one or more coatings of a refractory oxide as a diffusion barrier to prevent temperature "runaway" in catalytic combustion systems. See also U.S. Pat. No. 5,326,253 and U.S. Pat. No. 5,425,632, both of which teach the use of diffusion barrier layers for controlling the temperature of the catalyst. U.S. Pat. No. 5,202,303 and U.S. Pat. No. 5,183,401 teach monolithic combustion catalysts that also provide a measured temperature rise combustion.
In another known method for moderating catalyst temperature, premixed air and gas radially flow through alternate layers of catalytically-active and non-catalytic granulated beds. This design allows the alternating passive beds to cool the active beds while it limits the maximum temperature rise through the entire catalyst segment.